This invention refers to a compact knife unit, and is directed in particular to a pressure cutting unit in cutting machines for slitting sheets or webs of flexible material into strips, which require a high cutting precision which must be kept within very close tolerances.
In the slitting of a flexible web of material such as a web of paper, film, fabric or the like, it is common to use a series of compact or narrow slitting units, for example, of the general type disclosed in U.S. Pat. No. 3,977,284. In such a unit, a cast or fabricated metal body supports a knife holder for linear movement, and the knife holder has a slot for receiving a circular knife supported for rotation by a center shaft or pin. An air actuated piston moves the knife holder in a linear direction against a set of springs to press the circular knife into engagement with the web which is directed around a backup roll. The knife slitting units are individually clamped to a dovetail-shaped support bar and are sometimes positioned in closely spaced or adjacent relation in order to slit the web into narrow strips. The knife units currently in use, present a very low cutting precision, since it is not possible to control the cutting plane of the knife within close tolerances, as a result of the supporting system. Furthermore, it has been found desirable to provide for replacing a circular knife when the cutting edge becomes dull without removing the knife unit from the support bar. As shown in the above mentioned patent, the end portions of the shaft or pin supporting the knife are retained within slots defined by spring fingers formed as part of the knife holder. When it is desired to replace the knife, the opposite ends of the pin are pulled from the slots by deforming the spring fingers, and a new or resharpened knife is inserted into the knife holder with the support pin snap-fitting into the slots. Therefore, the spring fingers and the support of the knife by knife holders presently in use do not provide for a rigid and precision support for the knife due to the elasticity of the spring fingers and wear; if the spring fingers are made too rigid, it is difficult to remove and replace a knife.
For this reason, it is difficult to achieve the correct positioning of the knife unit, moreover, the operation of removing and repositioning a knife unit is very difficult to carry out on a cutting machine, due to the closeness of adjacent knife units. Consequently, the use of pressure knife units is not recommended and they are very rarely utilizable for the high-precision cutting of web materials used for technical purposes, for example, for slitting magnetic recording tapes, or tapes for computer printing units.
It has also been found desirable to minimize wear between the relative movable components or parts of the knife unit after extended use of the unit so that the circular knife does not wobble and its cutting edge remains precisely in track to assure close cutting tolerances. Such tolerances are especially important when the web is being slit into narrow strips each having a width within close tolerances.